Over the years various devices and methods have been developed for attending to fractured bones in an effort to achieve an effective healing of the fracture. The problem is ensuring that the splintered bone segments are "fixed" in a sufficiently secure position to prevent slippage or separation of the fractured segments during the healing process. The most common means for securing fractures is an external fixation device which extends into or through the bone fragments substantially orthogonal to the plane of the fracture. The fixation device is preferably directed inwardly into the fractured bone so as to pierce the outer cortex of the fractured bone, cross the medullary canal, and imbed its point in the opposite cortex. While such devices were historically used to temporarily stabilize the bone in preparation for a more permanent device, orthopedic physicians subsequently round these pins to be just as effective for stabilizing the bone during the entire healing process. Since then, these fixation devices have found general acceptance and are now widely used throughout the world.
Early fixation pins were configured with smooth, cylindrical shafts which were passed through pre-drilled holes. As these early pins had no threads about their shafts, the pins did not utilize a threaded engagement of the bone fragments. Rather, the pin was snugly fit within the pre-drilled hole merely to minimize slippage or separation. More contemporary fixation pins have employed a plurality of threads secured about a smooth, cylindrical shaft in order to improve upon the "fixed" nature of the pin within the fractured bone. Many were configured with self-tapping threads, thereby eliminating the need to tap a hole in the bone first. After a hole was drilled, the fixation pins were advanced into the hole, simultaneously tapping the sides thereof.
More modern fixation pins further improve upon the fixation process by eliminating the need to pre-drill a fixation hole in the bone. These pins consist of slenderly configured metal shafts which have a set of drilling teeth at a first end and a recess at the other end for receiving an operating tool. The tool assists in rotating the pin within the bone in order to advance or retract the pin therein. Many fixation pins of this type incorporate a pointed spade configuration at the drilling end wherein one or more obtuse, wedge-shaped, spade surfaces are positioned on opposite sides of a longitudinal axis with knife edges to scrape away the bone when the pin is turned. These fixation pins also employ self-tapping threads positioned along the shaft proximate to the sloping surfaces of the drilling teeth and extend for a distance sufficient to fix the bone on opposite sides of the fracture.
Despite the improvement over earlier devices, there are some disadvantages inherent with some of the self-drilling, self-tapping fixation pins presently available. First of all, it is difficult to construct drilling teeth out of suitably acceptable materials wherein the drilling teeth are sufficiently sharp to maintain the necessary pin advancement rate required by the self-tapping threads. An ineffectively slower advancement rate is achieved with simultaneous drilling of the bone hole in contrast to that achieved if a self-tapping pin were advanced in a pre-drilled hole. As a result, the threads tapped into the bore created by the drilling teeth are partially stripped away due to the slower advancement rate. To avoid this problem, some orthopedic surgeons, in treating a fractured bone, use only a self-tapping fixation screw and therefore pre-drill a bore into the bone. Following the creation of an acceptable bore, the orthopedic surgeon directs the fixation screw through the bone cortex on one side of the marrow, through the marrow, and then into the bore created in the bone cortex on the other side of the marrow.
A second problem develops with such slow advancement rates--that of excessively high temperatures resulting from the frictional engagement of the pin with the bone as the fixation pin is inserted. Often a portion of bone tissue, local to the bore, is exposed to excessive heat. Since bone cells are highly vulnerable to severe heat buildup and have been shown to die at temperatures as low as 105.degree. F., the use of self-drilling fixation pins sometimes results in a small region of dead bone tissue surrounding the pin, sometimes causing the pin to become unsecured. In such events, it is necessary for the stabilization process to be repeated to the discomfort and inconvenience of the patient.
While pre-drilling the fixation holes generally overcomes this problem, it results in a more time consuming surgical procedure. Not only is the additional pre-drilling step involved, but since the orthopedic surgeon is unable to see the interior portions of the pre-drilled hole after the drill bit is removed, the manner in which the fixation screw is advanced occurs solely by feel. The interior surfaces of the bone table, adjacent the marrow, are of a generally porous nature, making it difficult to slide a sharp point over the porous surface. Such circumstances further complicate the use of a self-tapping fixation pin not employing self-drilling means.
Recently there have been developed other variations of fixation pins, such as the cannulated fixation screw disclosed in U.S. Pat. No. 4,537,185 to Stednitz ("the '185 patent"). The fixation device disclosed in the '185 patent is a self-tapping, self-drilling, orthopedic fixation screw for use with a guide pin. The manner in which the cannulated fixation screw is used is significant because of its ability to overcome previous difficulties in accurately directing a fixation screw into a predetermined location. The conventional guide pin, having a relatively long thin shaft with threads and drilling teeth at one end, is first directed into the fractured bone so as to penetrate two or more of the fractured segments while maintaining a certain length of shaft outside the bone. The cannulated fixation screw includes an axial opening extending the entire length therethrough, wherein the opening has a diameter slightly larger than the diameter of the shaft of the guide pin. With such an arrangement, the fixation screw can be placed over the protruding end of the guide pin and rotated, by way of a separate advancing tool, into the bone area surrounding the guide pin. The drilling teeth on the fixation screw provide an opening large enough for the fixation screw to advance when the screw is rotated.
The cannulated fixation screw of the '185 patent further consists of a shaft having a plurality of straight axial flutes defining one side face of several drilling teeth, positioned at the forward end of the screw, and also the cutting face of the self-tapping threads is disposed proximal to the drilling teeth. The flutes consist of two perpendicular surfaces, a first surface being aligned coplanar with a diameter of the shaft. Separating the flutes are a plurality of lands defined by decreasing height threads which assist in exposing the first flute surface in order to provide a self-tapping surface. As with most conventional self-tapping screws, the fixation screw of the '185 patent includes a lead angle of incomplete thread heights, formed adjacent the drilling end of the screw, which provide a gradual increase in the height of the threads carved out in the bone up to the maximum thread height.
The device of the '185 patent is somewhat limited by its positioning of the cutting edge of the flutes in coplanar alignment with the diameter of the shaft. Such a configuration creates a neutral rake angle for tapping the hole through which the screw is directed. Neutral rake angles sometimes result in less accurately developed threads in the side walls of the bone while requiring greater force in advancing the fixation screw.
Another problem with cannulated fixation screws, such as that disclosed in the '185 patent, is the disposal of bone chips developed adjacent the drilling teeth. While theoretically the chips are removed from the bone by directing the chips outwardly through the flutes and the threads, as a practical matter, not all are removed. Some remain at the end of the bore during advancement of the fixation screw due to clogging of the flutes or threads, thereby impairing the drilling process.
Still another problem exists with the current fixation screws, wherein the self-tapping threads are positioned immediately adjacent the drilling teeth. The absence of a transition zone between the two features increases the likelihood of excessive heat build-up by precluding the ability of the surrounding bone to momentarily cool after being drilled before the self-tapping operation begins. Furthermore, the employment of self-tapping screws about a constant minimum diameter further increases heat build-up by making it more difficult to advance the fixation screw.
It would therefore be a novel improvement to provide a fixation device having self-tapping threads defined by a cutting face with positive rake angles in order to increase its effectiveness. It would be another improvement over the prior art to provide for tapered threads along a portion of the self-tapping threads to assist in the advancement process and contribute to the reduction of heat build-up. In addition, it would be a novel improvement to provide a means for more effectively removing bone chips away from the drilling end of the fixation screw in order to improve the drilling process. Furthermore, it would be a significant improvement over the prior art to provide a fixation screw which employs a transition zone between the drilling teeth and the self-tapping threads in order to further reduce the build-up of heat in the fractured bone. Finally, it would be a significant improvement to provide other means incorporated within the fixation screw to reduce heat buildup and thereby minimize bone cell destruction during the fixation process.